Switching mode power amplifiers are known from the state of the art as particularly efficient power amplifiers. Class-E amplifiers, for example, which have been presented in U.S. Pat. No. 3,919,656, are switching mode power amplifiers which can theoretically approach a power efficiency of 100%. Switching mode power amplifiers are used for example in transmitter architectures which require a low power consumption, like transmitter architectures employed for mobile devices.
While a switching mode power amplifier can be very efficient, it is inherently very non-linear, i.e. the amplitude of its output signals are not affected linearly by a change of the amplitude of its input signals within the regular operating range. On the other hand, a switching mode power amplifier does not alter significantly the phase of input phase modulated signals. Thus, switching mode power amplifiers are rather suited for amplifying constant-envelope phase modulated signals than amplitude modulated signals.
In some cases, signals that are to be amplified have no envelope variation in the first place. In some other cases, the switching mode power amplifier does not see the envelope variation, e.g. in the LINC system (LInear amplification using Non-linear Components) proposed by D. C. Cox of the Bell laboratories in “Linear Amplification with Nonlinear Components”, IEEE Transactions on Communications, COM-22, pp. 1942 to 1945, December 1974, or when using a bandpass pulse position modulation (PPM). In the latter case, the structure comprising the switching mode power amplifier as a whole takes care of generating the correct envelope for the transmitter output signal.
A structure employing a switching mode power amplifier is given for example with a conventional envelope elimination and restoration (EER) transmitter. In such an EER transmitter, a constant-envelope phase-modulated radio-frequency signal is input to the switching mode power amplifier for amplification. The envelope is then restored in the switching mode power amplifier by varying its supply power.
In most applications, it is required that the average power level of the signals output by a power amplifier can be controlled, possibly even over a very large dynamic range. In a conventional power control of the output power level of a power amplifier, a variable gain amplifier (VGA) is arranged in front of the power amplifier, which pre-amplifies the input signal according to the desired output power level. Since a switching mode power amplifier is inherently very non-linear, however, a conventional power control is not suitable for a switching mode power amplifier. On the other hand, the dynamic range that can be achieved by varying the power supply to the switching mode power amplifier mentioned above is restricted by a lower limit. This lower limit results from a leakage of an input radio frequency signal through the transistor of the switching mode power amplifier due to its parasitic capacitances.
There are various documents dealing with the power control of switching mode power amplifiers.
F. H. Raab, B. E. Sigmon, R. G. Myers and R. M. Jackson, for instance, discuss in the document: “L-Band Transmitter Using Kahn EER Technique”, IEEE Transactions on Microwave Theory and Techniques, Vol. 46, No. 12, December 1998, the use of non-linear power amplifiers in EER architectures and mention that the drive level to the power amplifier should be adjusted in order to maintain the output saturation at all supply voltage values.
Document WO 98/49771 proposes to treat voltage and bias current control of the power amplifier in order to improve the efficiency at low power levels.
Document U.S. Pat. No. 6,323,731 proposes to use a dynamic bias control for the power amplifier in order to widen the output power range in case where the power control is accomplished through the power amplifier supply voltage.
None of these documents, however, enables a linear power control for a switching mode power amplifier over a very large dynamic range while preserving the efficiency of the amplifier.